Hey nitai, I sailed at manly for years (I now sail at wello since I now live there). I have fished moreton bay all of my life.
There are many big sharks in the bay, tangalooma used to be a whaling station and it was here I believe that the world record white pointer was caught that the story refers to. When we went to St Helena as kids they told us they used to try to attract the sharks to deter prisoners from swimming to the mainland. I have caught heaps of sharks, mostly small ones like pictured but many I have had no chance of landing on some pretty heavy tackle. Dont be put off by the number of small sharks though think of these just like any other fish and are doing us a favour by keeping the bay reasonably clean.
There are situations and places to avoid though, where that woman swimming at amity pt was is one of them. I would never swim in/near the rainbow channel ever (I have actually been fishing at amity when less than 3m away big sharks have come near the surface and scared the crap outta me cause that is damn close when you are sitting by or standing in the water). Think about it this way, big sharks cover a huge area but they will stay where most of the food is. Everything that comes into moreton bay (big sharks, whales, schools of fish etc)comes through 1 of 2 places north of moreton which is quite a large area or between moreton and straddie which is quite narrow condensed area.
Avoid the shipping channel, big sharks traditionally follow the ships and eat all the garbage. Avoid shallow areas next to big drop offs and areas where there is deep-shallow-deep ie the bank that goes out from the point at Manly, dont avoid it like the plague just be wary not to stand out there for half and hour taking a break-it never stopped some of us though(there is a gutter that runs out on the northern side of that bank which is called shark alley/gutter). Most attacks happen in shallow water due to the displacement of water by the shark and natural tendancies to chase things which dont swim to well on the land. I dont know if you've ever seen the dolphins working the schools of fish into the beach down there at Manly, it is pretty cool.
Avoid crab pots and to some extent be aware of the time you spend in the water during crabbing season. Avoid sailing at sunrise/sunset, fishing though we would tend to catch sharks just after the sun has gone down... too dark to sail then. Avoid sailing just after heavy rain, there are more implications than just sharks here hitting a tree floating just under the water tends to put a damper on your day as does stepping on a catfish or bullrout that get flushed into the bay. A word of warning, there used to be a huge tiger shark that used to come into the bay for a week or so every year I dont know if it still does but it used to be around d'arcy light and come down into the bay at manly. We always used to hear if it was around or see it, most people still sailed.
After all that just go out and sail, most people I know suffer some sort of shark phobia. As said a million times before you have greater chance of getting killed by being hit by a car, a boat, another sailboarder or 100x greater chance of a kite landing on your head

. Its all a game of odds, when your time is up your time is up. Dont believe all the crap in the media, a so called shark expert said bull sharks are more prevalent in the bay after rain since they get sick from fresh water... this is strange since they can and do live in fresh water (fish get ill/sluggish from fresh water, easy food for sharks)- I hate the media.
If you are ever sailing at manly or wello and see me out come up and say hi, I have either MK7 or AUS357 sail numbers so pretty noticeable. I dont sail manly too much unless I can drag my mate out that lives down there. I sailed there saturday for the first time in ages, was good fun a little gusty but good for a change of scenery.